This invention relates to a method of preparing a polybenzoxazine and a method of coating a device using such a polybenzoxazine.
Electronic devices such as circuit boards, semiconductors, transistors, and diodes are often coated with materials such as epoxy resins for protection. Such coating materials are often cured on the surface of an electronic device by heat. But electronic devices often are sensitive to heat, and too much heat may adversely affect the performance of a device. Further, if the coating material shrinks or expands significantly in response to heat, the device it coats may be warped. Thus, it is desirable to develop methods for curing coating materials at relatively low temperatures in short time periods and to develop coating materials that have a near-zero volume change upon heat treatment so as to minimize the possiblities of damaging the coated devices.
In general, the invention relates to methods of preparing a polybenzoxazine (PBO) composition at relatively low temperature in short time periods. The methods can be used, for example, to provide a coating on electronic devices such as circuit boards and semiconductors. The preferred PBO compositions have high glass transition temperature, good electrical properties (e.g., dielectric constant), low flammability, and a near-zero percent shrinkage and expansion upon demolding, postcuring, and cooling.
In one aspect, the invention features a method of preparing a PBO including heating a molding composition having a benzoxazine and a heterocyclic dicarboxylic acid to a temperature sufficient to cure the molding composition, thereby forming the PBO.
In another aspect, the invention features a method of preparing a PBO including heating a molding composition having a benzoxazine and a catalyst to a temperature in the range of about 150xc2x0 C. to about 250xc2x0 C. to cure the molding composition in about 1 minute to about 5 minutes, thereby forming the PBO. Preferably, the method can be carried out in the range of about 160xc2x0 C. to about 210xc2x0 C. in about 2 minutes to about 4 minutes.
In another aspect, the invention features a method of coating a device including heating a molding composition having a benzoxazine and a heterocyclic dicarboxylic acid to a temperature sufficient to cure the molding composition, thus forming a PBO which coats a surface of the device. The device can be an electronic device such as a semiconductor or a circuit board.
In another aspect, the invention features a method of coating a device including heating a molding composition including a benzoxazine to a temperature sufficient to cure the composition, thereby forming a polymer composition. The polymer composition forms on a surface of the device, and results in essentially no warpage of the device after post curing the molding composition. The device can be a semiconductor or a circuit board.
The invention also relates to a molding composition including a benzoxazine and a heterocyclic dicarboxylic acid; a polymer composition including a polybenzoxazine and a heterocyclic dicarboxylic acid; and a device coated with a polymer composition including a polybenzoxazine and a heterocyclic dicarboxylic acid.
The invention also relates to a benzoxazine-containing molding composition that has a post cure volume change of less than 0.15%, preferably less than 0.10%, and more preferably less than 0.05%. A post cure volume change includes shrinkage or expansion and is measured according to the procedure described subsequently in the application.
The invention also relates to a benzoxazine-containing molding composition. The composition, when applied on a FR-4 board, results in essentially no warpage of the device after post cure. Warpage is measured according to the procedure described subsequently in the application.
The invention still further relates to a device coated with a polymer composition including a polybenzoxazine, an epoxy resin, and a phenolic resin.
The heterocyclic dicarboxylic acid includes an X,Y-containing heterocyclic moiety and a dicarboxylic acid moiety which is bonded to the X,Y-containing cyclic moiety. The heterocyclic dicarboxylic acid is of formula (I): 
X is N, O, or NRa, where Ra is hydrogen, alkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, aralkyl, or heteroaralkyl. Preferably, X is N or O. More preferably, X is N. Y is O, S, NRb, or C(Rc) (Rd), where Rb is hydrogen, alkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, aralkyl, or heteroaralkyl, and where each of Rc and Rd, independently, is hydrogen, alkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, aralkyl, heteroaralkyl, alkoxy, hydroxyl, hydroxylalkyl, carboxyl, halo, haloalkyl, amino, aminoalkyl, nitro, cyano, alkylcarbonyloxy, alkyloxycarbonyl, alkylcarbonyl, formyl, aminocarbonyl, alkylcarbonylamino, alkylsulfonylamino, aminosulfonyl, aminocarbonyloxy, or alkyloxycarbonylamino. Preferably, Y is O or S. More preferably, Y is S. Z is a bond, S, O, or NRe, where Re is hydrogen, alkyl, cycloalkyl, heterocyclo-alkyl, aryl, heteroaryl, aralkyl, or heteroaralkyl. Preferably, Z is a bond or S. More preferably, Z is a bond. Each of R1, R2, and R3, independently, is hydrogen, alkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, aralkyl, heteroaralkyl, alkoxy, hydroxyl, hydroxylalkyl, carboxyl, halo, haloalkyl, amino, aminoalkyl, alkylcarbonyloxy, alkyloxycarbonyl, alkylcarbonyl, alkylcarbonylamino, aminocarbonyl, alkylsulfonylamino, aminosulfonyl, sulfonic acid, or alkylsulfonyl. R1 and R2, optionally, can join together to form a cyclic moiety. Likewise, R2 and R3, optionally, can join together to form a cyclic moiety. The cyclic moiety formed by joining R1 and R2, or R2 and R3 is substituted or unsubstituted cycloalkyl, substituted or unsubstituted heterocycloalkyl, substituted or unsubstituted aryl, or substituted or unsubstituted heteroaryl. Preferably, R1 and R2 join together to form a cyclic moiety. More preferably, the cyclic moiety is aromatic. An example of such an aromatic cyclic moiety is a benzene ring. The cyclic moiety can be further substituted by groups such as alkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, aralkyl, heteroaralkyl, alkoxy, hydroxyl, hydroxylalkyl, carboxyl, halo, haloalkyl, amino, aminoalkyl, alkylcarbonyloxy, alkyloxycarbonyl, alkylcarbonyl, alkylcarbonylamino, aminocarbonyl, alkylsulfonylamino, aminosulfonyl, sulfonic acid, or alkylsulfonyl. m is 0 or 1. Preferably, m is 0. When m is 0, R3 and the carbon atom to which R3 bonded are deleted, and Y is directly bonded to the carbon atom to which R2 is bonded. n is 0, 1, 2, 3, 4, 5, or 6. Preferably, n is 1.
The X,Y-containing heterocyclic moiety can be saturated, unsaturated, or aromatic. In one embodiment, the X,Y-containing heterocyclic moiety can be a furan, a thiophene, a thiazole, an oxazole, an imidazole, a pyridine, a piperidine, or a pyrimidine. Preferably, the X,Y-containing heterocyclic moiety is a thiazole.
Examples of heterocyclic dicarboxylic acid of formula (I) include 2-(2-benzthiazolyl)-succinic acid and (2-benzthiazolylthio)-butanedioic acid, available from Ciba Geigy under the trade name IRGACOR 252LD and IRGACOR 252FC, respectively.
It should be noted that heterocyclic dicarboxylic acid can form an acid anhydride. Such an acid anhydride is also within the scope of this invention.
As used herein, alkyl is a straight or branched hydrocarbon chain containing 1 to 8 carbon atoms. Examples of alkyl include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, and 2-methylhexyl.
As used herein, cycloalkyl is a cyclic alkyl group containing 3 to 8 carbon atoms. Some examples of cycloalkyl are cyclopropyl, cyclopentyl, cyclohexyl, cycloheptyl, adamantyl, and norbornyl. Heterocycloalkyl is a cycloalkyl group containing 1-3 heteroatoms such as nitrogen, oxygen, or sulfur. Examples of heterocycloalkyl include piperidinyl, piperazinyl, tetrahydropyranyl, tetrahydrofuryl, and morpholinyl.
As used herein, aryl is an aromatic group containing 6-12 ring atoms and can contain fused rings, which may be saturated, unsaturated, or aromatic. Examples of an aryl group include phenyl, naphthyl, biphenyl, phenanthryl, and anthracyl. Heteroaryl is aryl containing 1-3 heteroatoms such as nitrogen, oxygen, or sulfur and can contain fused rings. Some examples of heteroaryl are pyridyl, furanyl, pyrrolyl, thienyl, thiazolyl, oxazolyl, imidazolyl, indolyl, benzofuranyl, and benzthiazolyl.
Amino groups can be unsubstituted, mono-substituted, or di-substituted. It can be substituted, for example, with groups such as alkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, aralkyl, or heteroaralkyl. Halo refers to fluoro, chloro, bromo, or iodo.
As used herein, a cyclic moiety refers to a 5- to 6-membered cycloalkyl, heterocycloalkyl, aryl, or heteroaryl moiety. A cyclic moiety can also be fused rings and can be formed from two or more of the just-mentioned groups. Each of these moieties is optionally substituted with alkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, aralkyl, heteroaralkyl, alkoxy, hydroxyl, hydroxylalkyl, carboxyl, halo, haloalkyl, amino, aminoalkyl, alkylcarbonyloxy, alkyloxycarbonyl, alkylcarbonyl, alkylcarbonylamino, aminocarbonyl, alkylsulfonylamino, aminosulfonyl, sulfonic acid, or alkylsulfonyl.
As used herein, a molding composition refers to a composition having a benzoxazine monomer that can form a PBO polymer composition of this invention.
As used herein, a molding composition is cured when it forms a good cull cure (i.e., strong and not brittle).
Other features and advantages of the invention will be apparent form the description of the preferred embodiments thereof, and from the claims.
The preferred method involves heating a molding composition including a benzoxazine and a heterocyclic dicarboxylic acid having formula (I) at from 150xc2x0 C. to 250xc2x0 C., preferably from 160xc2x0 C. to 210xc2x0 C., for 1 to 5 minutes, preferably for 2 to 4 minutes, to form a PBO composition. The preferred PBO composition contains a PBO and a heterocyclic acid catalyst, and optionally an epoxy resin, a phenolic resin, and a second acid catalyst.
Suitable benzoxazine monomers can be prepared by condensing two equivalents of formaldehyde with one equivalent of a primary amine (e.g., methylamine and aniline) and reacting with one equivalent of a phenol (e.g., bisphenol-A). For reference, see, e.g., Burke et al., J. Org. Chem. 30(10), 3423 (1965). A schematic representation of this reaction is reproduced below. The groups R, Rxe2x80x2, and Rxe2x80x3 are not particularly limited and can be hydrogen or other suitable substituents such as alkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, aralkyl, heteroaralkyl, alkoxy, hydroxyl, hydroxylalkyl, carboxyl, halo, haloalkyl, amino, aminoalkyl, alkylcarbonyloxy, alkyloxycarbonyl, alkylcarbonyl, alkylcarbonylamino, aminocarbonyl, alkylsulfonylamino, aminosulfonyl, sulfonic acid, or alkylsulfonyl. 
The benzoxazine monomers can then undergo a ring opening thermal polymerization reaction in the presence of a heterocyclic dicarboxylic acid catalyst of formula (I) to form a PBO as shown in the reaction below. 
Because of the nature of the polymerizing reaction, addition of each monomer to the propagating polymer only takes place at the ortho or para position of the benzene ring. To ensure that the PBO propagates only at the ortho position of the benzene ring, the ortho position is not substituted, whereas the para position is substituted. Each of the two meta positions of the benzene ring, independently, can be substituted or unsubstituted. However, although not directly involved in the polymerization, the meta-substituents (i.e., Rxe2x80x2) can potentially affect the reaction by steric hindrance. Bi-functional benzoxazine monomers (e.g., benzoxazine monomers prepared from bisphenol-A, formaldehyde, and aniline) can also be employed in the polymerization reaction.
The preferred weight percent of the benzoxazine monomer present in the molding composition ranges from 5.0% to 20.0%, more preferably from about 10.0% to about 15.0%, based on the total weight of the composition.
The benzoxazine-containing molding compositions can be prepared by any conventional methods. For example, the ingredients (including resins and other additives) can be finely ground, dry blended, densified on a hot differential roll mill, and then followed by granulation. The molding composition, as described above, can be used for coating electronic devices such as semiconductors or circuit boards. The prepared compositions can be molded by any suitable molding apparatus. An example of such an apparatus is a transfer press equipped with a multi-cavaity mold. For more detail on methods for preparing molding compositions and for coating electronic devices, see U.S. Pat. No. 5,476,716.
The heterocyclic dicarboxylic acid of formula (I) can be obtained from commercial sources or can be prepared by known methods. For example, 2-(2-benzthiazolyl)-succinic acid and (2-benzthiazolylthio)-butanedioic acid are commercially available from Ciba Geigy under the tradenames IRGACOR 252LD and IRGACOR 252FC, respectively. A general method for preparing a heterocyclic dicarboxylic acid in which Z is a bond of formula (I) is shown in the following scheme. 
Referring to the above scheme, a protected dicarboxylic acid is first deprotonated by a base such as sodium butoxide. An example of a protecting group for the dicarboxylic acid is an ester such as ethyl ester. The deprotonated acid then reacts with a X,Y-containing heterocyclic compound having a suitable leaving group such as Cl to form a protected heterocyclic dicarboxylic acid which can then be deprotected to yield the acid catalyst of formula (I).
For compounds of formula (I) wherein Z is NH, it can be prepared by reacting a compound of formula (II) with a compound such as aspartic acid (n=2) or glutamic acid (n=3). Compounds of formula (I) wherein Z is O or S can be prepared in a similar manner.
The preferred weight percent of the heterocyclic dicarboxylic acid that can be used in catalyzing the preparation of PBO ranges from 0.3% to 5.0%, more preferably from 1.0% to 3.0%, based on the total weight of the composition.
The second acid catalyst can be a dicarboxylic acid or an acid anhydride. An example of such an acid anhydride is benzophenone tetracarboxylic acid dianhydride. The dicarboxylic acid is preferred to be an aliphatic dicarboxylic acid of the formula HOOCxe2x80x94(CH2)pxe2x80x94COOH where p is 1-8. An example is adipic acid (i.e., p is 4). The molding composition may include, for example, 0.1 to 5.0 wt %, preferably 0.3 to 3.0 wt % of the second acid catalyst.
An example of an epoxy resin is epoxy cresol novalac. The molding composition may include, for example, about 0.5 wt % to about 7.0 wt %, preferably about 1.5 wt % to 3.5 wt %, of the epoxy resin.
An example of a phenolic resin is phenolic novalac. The molding composition may include, for example, 0.1 wt % to 3.0 wt %, preferably 0.3 wt % to 1.5 wt %, of the phenolic resin.
Below are some examples of other additives that can be included in the molding composition and the preferred ranges of their weight percent in the composition:
(1) A flame retardant such as a brominated epoxy novolac flame retardant (e.g., BREN, available from Nippon Kayaku). The preferred molding composition can contain up to 3.0 wt %, more preferably, 0.1-1.0 wt % of a flame retardant.
(2) A flame retardant synergist such as Sb2O5 or WO3. The preferred molding composition can contain up to 3.0 wt %, more preferably, 0.25-1.5 wt % of a flame retardant synergist.
(3) A filler such as silica, calcium silicate, and aluminum oxide. The preferred molding composition can contain 70-90 wt %, more preferably, 75-85 wt % of a filler.
(4) A colorant such as carbon black colorant. The preferred molding composition can contain 0.1-2.0 wt %, more preferably, 0.1-1.0 wt % of a colorant.
(5) A wax or a combination of waxes such as carnauba wax, paraffin wax, S-wax, and E-wax. The preferred molding composition can contain 0.1-2.0 wt %, more preferably, 0.3-1.5 wt % of a wax.
(6) Fumed silica such as aerosil. The preferred molding composition can contain 0.3-5.0 wt %, more preferably, 0.7-3.0 wt % of fumed silica.
(7) A coupling agent such as the silane type coupling agent. The preferred molding composition can contain 0.1-2.0 wt %, more preferably, 0.3-1.0 wt % of a coupling agent.
The preferred molding compositions cure in from 1 minute to 5 minute, more preferably, from 2 minutes to 4 minutes. To determine the time for curing (i.e., minimum time needed for forming a good cull cure), the molding composition is placed in the mold press at 190xc2x0 C. and is inspected after a pre-set period of time (e.g., 3 minutes). If a good cure (i.e., strong and not brittle) is formed, the experiment is repeated with a shorter period of press time until the minimum time period is determined.
The preferred molding compositions having a less than 0.15%, preferably less than 0.10%, post cure volume change includes a benzoxazine, and can further include an epoxy resin and a phenolic resin. The post cure volume change of a composition is measured according to the following steps which are based on a standard test procedure (ASTM D955-73) provided by the American Society for Testing and Materials.
(1) Prepare a single bar, single-cavity positive compression mold of inner dimensions 12.7 mmxc3x9712.7 mmxc3x97127 mm; a compression hydraulic press that delivers 1000 psi to the composition in the mold; and gages accurate to 0.02 mm for measuring the composition;
(2) fill the mold with a molding composition in such a way that there is no appreciable lateral movement of the composition during the molding process;
(3) condition the composition at 23xc2x12xc2x0 C. and 50xc2x15% relative humidity for 24 hours;
(4) mold the composition at 190xc2x0 C. and 1000 psi for 4 minutes;
(5) post cure at 175xc2x0 C. for 4 more hours;
(6) allow the composition to cool to room temperature;
(7) measure the length of the composition to the nearest 0.02 mm at room temperature (i.e., 23xc2x12xc2x0 C.);
(8) calculate the % volume change by subtracting the dimension of the molded composition from the corresponding dimension of the mold cavity in which it was formed, dividing the difference by the latter, and multipling the quotient by 100%.
Note that the test is repeated three times so that an average volume change can be obtained.
When the preferred benzoxazine-containing molding compositions is applied on a FR-4 board, essentially no warpage of the device is obverved. Warpage results as the volume of the molding composition that is on the surface of one side of the FR-4 board changes significantly upon demolding and cooling, thereby bending the coated board. To measure warpage, a FR-4 board of the dimensions 89 mmxc3x9789 mmxc3x970.36 mm is used. A molding composition is applied and molded to one side of the FR-4 board by a transfer press to form a coating of the dimension 79.5 mmxc3x9779.5 mmxc3x971.77 mm at 190xc2x0 C. and 1000 psi for 4 minutes. The coated FR-4 board is then post cure at 175xc2x0 C. for 4 more hours at the same pressure. After cooling to room temperature, the FR-4 board is placed on a flat surface with one corner being depressed. The distance between the diagonal corner of the board and the flat surface where the board is sitting is measured. An essentially zero warpage refers to a distance of no more than 2 mm. An FR-4 board is only used as an example. Other electronic devices such as BT boards (available from Herco Company, Los Angeles, Calif.) can also be coated with the preferred benzoxazine-containing molding composition without suffering from warpage.
The preferred molding compositions having a less than 0.15% volume change can include a heterocyclic dicarboxylic acid, a second dicarboxylic acid, and other additives such as fillers, colorants, flame retardants, flame retardant synergists, wax, fumed silica, and coupling agents as described above.
The preferred polymer composition may include 5.0% to 20.0%, more preferably from 10.0% to 15.0% of a PBO.
The preferred PBO""s have molecular weight ranges from 200 to 3000, more preferably from 400 to 2000; a glass transition temperature ranges from 140xc2x0 C. to 220xc2x0 C., more preferably from 150xc2x0 C. to 210xc2x0 C.; a dielectric constant ranges from 3.2 to 4.0, more preferably from 3.4 to 3.5; and volume change (including shrinkage and expansion) of no more than 0.15%, preferably no more than 0.10%, and more preferably no more than 0.05%.